I've got a Bushmaster A2 that I want to convert to a flat-top so I can run a scope. Is it a good idea to put a DPMS tri-mount upper on a Bushmaster? Or should I get a Bushmaster V-Match upper and spend more for riser blocks?

Some people here look down on anything that left the factory less than a complete weapon. Others are adherents to one brand or the other (or two or three) to the exclusion of others.
I have mixed and matched various brands for years to come up with exactly what I want for each rifle, and I have been well-served by this approach. Stick with quality components and you shouldn't have problems. Bushmaster and DPMS are both good brands - if you like the layout, value, etc. of the DPMS upper, then go for it! Good luck!
edited to add:
The above having been said, I usually get around to putting a lower of the same brand with any given upper eventually, then getting a new upper for the newly "abandoned" lower - but I buy, sell, and trade ARs more than most. I eventually match up brands because of some personal O/C issues - not because of compatibility problems or anything like that.

If the only tool you have is an AR-15, then every problem looks like a TARGET.(With apologies to Maslow...)

I have Colt barrel with Bushmaster upper, I have Colt upper with Colt barrels, and I have Bushmaster upper with Match stainless barrel, factory or mixed they all work flewlessly. as long as you are buying quality parts (DPMS and Bushmaster are quality parts), you should have no problem what so ever.